This invention relates to an optical shutter array utilizing electrooptic effect, and more particularly, to such an optical shutter array characterized by low driving voltage and simple wiring, and a method for making the same.
In recent years, electrooptic materials or materials exhibiting an electrooptic effect have been developed. The electrooptic effect is the phenomenon that indices of refraction are changed by an applied DC or low frequency (as compared with light frequency) electric field. As a typical example of these electrooptic materials is known a transparent ceramic PLZT material having a composition of 9/65/35, that is, 9 atomic percents of La and a molar ratio of PbZrO.sub.3 /PbTiO.sub.3 of 65/35.
The PLZT finds one application as an optical shutter. The optical shutter is of the structure wherein a plate-shaped PLZT element having a pair of spaced-apart planar electrodes formed on one major surface thereof is interposed between a polarizer and an analyzer which are located such that their directions of polarization are perpendicular to each other. Light incident on the polarizer can be controlledly transmitted and interrupted by turning on and off a voltage applied to the electrodes.
If a number of such optical shutters could be integrated into an optical shutter array having a high packing density, then a subtle image would be produced with a high resolving power. Instant efforts to accomplish such an array have encountered several problems, among which important are the following two.
(1) A high driving voltage must be applied in order to increase the contrast of outgoing light.
(2) Wiring is difficult.
These problems will be discussed in detail. The first problem is the neccesity of applying a high driving voltage for increased contrast of outgoing light. It is now assumed that an optical shutter has an electrooptic element sandwiched between a polarizer and an analyzer. When monochromatic light having an intensity I.sub.0 and a wavelength of .lambda. is incident on the polarizer, the analyzer transmits light having an intensity I represented by the formula: ##EQU1## where L is the thickness of the electrooptic element across which the light passes, that is, an effective length of light path, and .DELTA.n is birefringence.
If the birefringence is chosen so as to be .DELTA.nL=.lambda./2 in formula (1), then I=I.sub.0, that is, the light transmitted by the system has the maximum intensity I. The birefringence .DELTA.n can be changed by the magnitude of an electric field applied to the element. For example, when the element has a quadratic electrooptic effect, the birefringence .DELTA.n changes in proportion to the square of an electric field strength E as represented by the following formula: ##EQU2## where R is an electrooptic coefficient and n is an index of refraction. Then, the intensity I of the transmitted light is changed from zero to the maximum I.sub.0 by applying a rectangular waveform voltage having a magnitude E: ##EQU3## where K=-1/2n.sup.3 R.
The voltage V that provides E satisfying formula (3) is called half-wave voltage and designated V(1/2). The half-wave voltage is correlated to the driving voltage of an optical shutter array.
Therefore, as the electric field strength E is increased, the birefringent .DELTA.n is increased and hence, the transmitted light intensity I is increased. Also, as the effective length of light path L is increased, the half-wave voltage is lowered so that an increased I is available with a low driving voltage.
With increased I, the contrast of light transmitted by turning on an off of an applied voltage is enhanced. Then, an optical shutter array can be driven with a lower voltage if the effective length of light path is increased.
A prior art optical shutter is illustrated in FIG. 3. Electrodes 4 are formed on an electrooptic material such as PLZT as planar electrodes having a transverse width W by a metallizing or similar technique. In order to increase the effective length of light path L the electrode width W must be increased. However a certain upper limit is imposed on the width W. In order to increase the packing density of an optical shatter array to produce a precise image having a high resolving power, the size of one picture element must be reduced and hence, the electrode width W is required to be 50 .mu.m or less, and sometimes 5 .mu.m or less. For this reason, a prior art optical shutter array is designed so as to achieve an increased I by setting a high electric field strength E. Such a higher field strength E leads to a greater energy consumption, leaving many problems in the design of a device. It is said that formula (2) ceases to be true when the field strength E exceeds about 15 to 25 kV/cm.
In order that PLZT optical shutter arrays find a commercial application, the necessary driving voltage must be lowered. Since currently available single shutters have a half-wave voltage V(1/2) of about 280 V, it is strongly desired that optical shutter arrays have a half-wave voltage V(1/2) of about 80 V or lower.
For the purpose of lowering the driving voltage, it is proposed to replace the planar electrodes by channel electrodes as shown in FIGS. 4 and 5 (see Japanese Patent Application Kokai No. 58-82221 and Kurita et al., Proceedings of the 1985 Autumn Japan Applied Physics Society Meeting, 44). The term "channel electrode" used herein is an electrode prepared by forming a channel in a transparent substrate of electrooptic material and filling the channel with an electroconductive material.
In the structure of FIG. 4, the channel electrodes 4 are prepared by wet etching a PLZT substrate 3 with a strong acid such as HCl by photolithography. In the structure of FIG. 5, the channel electrodes 30 are prepared by machining channels 19 in a PLZT substrate 3 by means of a dicing saw. In either case, the channel electrodes are intended to increase the effective length of light path L to lower the half-wave voltage V(1/2).
With respect to these channel electrodes the channels that are formed by wet etching can only have a depth of approximately 10 .mu.m at the maximum. It is almost impossible to form channels to a depth of 50 .mu.m or more by wet etching. The effective length of light path cannot be further increased beyond the limit determined by the maximum channel depth.
Also in the structure of FIG. 5 wherein channels are machined using a dicing saw and a metal such as gold is deposited to form electrodes by sputtering, it is difficult to deposit a sufficient amount of the metal on the wall of the channels if the channels are narrow and deep. As will be demonstrated later by experimental data in Examples, channels having a width of 40 .mu.m resulted in an effective length of light path corresponding to a depth of approximately 20 .mu.m. When the depth of channels exceeds 50 .mu.m at a width of 40 .mu.m, the metal could not be deposited in a sufficient amount to form electrodes. Undesirably, the driving voltage could not be sufficiently lowered.
In both cases of the planar electrodes and the channel electrodes formed by wet etching, since the electrodes are rather present in a surface portion and a relatively high voltage must be applied, the electric field defined between the electrodes is deeply curved in the PLZT substrate 3 as shown by broken lines 11 in FIGS. 3 and 4. When a shutter array is formed with one picture element defined by each pair of electrodes, there is the likelihood of so-called cross-talk that the curved electric field associated with one picture element overlaps the electrodes associated with adjacent picture elements, resulting in a deteriorated SN ratio.
The second problem is the difficulty of wiring.
FIGS. 22 and 23 illustrate the prior art structures of wiring connection of electrodes on an optical shutter array. In the optical shutter array generally designated at 1, a substrate 3 having a plurality of parallel electrodes 4 is attached to a support 2. The support 2 is provided with a corresponding plurality of tapping electrodes 27. The electrodes 4 are connected to the corresponding tapping electrodes 27 through wires 31, respectively.
The process of bonding the wires 31 to the electrodes 4 and 27 is troublesome and time consuming because many connections must be made. There is the risk of disconnection due to probable lack of adherence of wire bonding, also causing a loss of productivity. Particularly when the electrodes 4 are of the channel type, only a narrow area is available on the electrodes for wire bonding so that the wire bonding operation is very difficult.
In the optical shutter array shown in FIG. 23 wherein the longitudinal direction of the electrodes 4 is aligned with the direction of incident light as shown by a solid arrow, the wires 31 connecting the electrodes 4 and the tapping electrodes 27 extend across the light path because of the location of the electrodes relative to the light path. Then, some wires 31 interrupt or scatter the incident light into stray light. The wires 31 have the likelihood of deteriorating the performance of the optical shutter array.
In addition to the first and second problems explained above, integrating a number of optical shutters into an optical shutter array having closely spaced electrodes gives rise to the problem that those portions of the electrodes exposed on the PLZT substrate surface undergo an air discharge upon application of an electric field of about 2 V/.mu.m so that the exposed electrode portions are readily damaged and thus deteriorated during the operation of the shutter array.